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The red giant star Betelgeuse in the constellation Orion - famed as the home sun of Hitchhiker's Guide to the Galaxy characters Zaphod Beeblebrox and Ford Prefect - is shrinking rapidly. Astronomers say that it has shrunk by 15 per cent since 1993, by which they mean that it actually did so in the mid 16th century. It may, in fact, already have exploded.

Betelgeuse, before it shrank, was thought by astro boffins to be so large that if it were placed in the middle of our solar system, Jupiter - out beyond the asteroid belt in reality - would lie inside it. Now it has shrunk by a distance equal to the orbital radius of Venus.

"To see this change is very striking," said Charles Townes, UC Berkeley emeritus physics prof and Nobel Prize winner. "We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size."

The huge star, one of the brightest in the sky, is thought to lie about 430 light years from our solar system, so the changes being observed now actually occurred in 1579 AD. Many boffins believe that Betelgeuse is so vast that it's liable to go supernova - that is, blow up with stupendous, galaxy-shaking force - within a millennium or so. Indeed, it might already have exploded at some point in the last 430 years, in which case the flash wouldn't yet have reached us.

"We do not know why the star is shrinking," says Edward Wishnow of US Berkeley's space sciences lab. "Considering all that we know about galaxies and the distant universe, there are still lots of things we don't know about stars, including what happens as red giants near the ends of their lives."

The name Betelgeuse, like many names of important stars, is derived from Arabic - Arabian civilisation having done much of the early astronomy carried out on Earth. Its underlying Arabic phrase has been translated variously into English, with most sources agreeing that the name is meant to refer to the star's position within the figure of Orion the Hunter - "the central one", to the old time stargazers. Indeed, it's also known as Alpha Orionis.

Some say that "Betelgeuse" refers in fact to Orion's "hand" or "arm". However an awful lot of people go with "armpit of the Central One", and it's certainly true that the star is located in the region more of the figurative Orion's shoulder than his actual arm or hand.

Much more famously, Betelgeuse - specifically the notional planet Betelgeuse V - was said to be the home world of two-headed, three-armed renegade Galactic President Zaphod Beeblebrox and his childhood friend and 'semi-cousin' Ford Prefect, in the Hitchhiker's Guide to the Galaxy stories by Douglas Adams.

Fans will be hoping that the recent shrinkage of Zaphod's sun doesn't mean that, in fact, his homeworld was destroyed hundreds of years before Earth's abrupt demolition to allow construction of a hyperspace bypass. ®